(a) Technical Field
The present invention relates to a polyisocyanate compound and a process for producing the same. More specifically, the present invention relates to a novel polyisocyanate compound useful as a starting material for an optical material which has excellent optical characteristics such as a high refractive index and low dispersion, and a process for producing the same with good efficiency.
Moreover the present invention relates to an optical material and an optical product using the same. More specifically, it relates to an optical material which has excellent optical characteristics such as high refractive index, low dispersion, excellent transparency, lack of optical distortion and the like, and which is also good in heat resistance, solvent resistance and weatherability, and to an optical product formed of this optical material, such as a lens, a prism, optical fibers, a substrate for a recording medium, a filter, a glass, a vase and the like.
(b) Related Art
In comparison with glass, plastics are lightweight, hard to break, and easy to dye. Therefore, plastics have been applied, in recent years, to optical use in various lenses and the like. As optical plastic materials, polyethylene glycol bisallyl carbonate (CR-39) and polymethyl methacrylate (PMMA) have been generally used. However, these plastic materials have a refractive index of 1.50 or less. Therefore, when they are used as lens materials, for example, the thickness of the lenses has to be increased with increasing lens power. Consequently, not only is the superiority of plastics lightweight materials impaired, but thick lenses have undesirable aesthetic properties. Furthermore, concave lenses in particular have been problematic in that the thickness (edge thickness) of the surrounding portion of the lens is increased and birefringence or chromatic aberration is liable to occur.
Therefore, in order to be able to decrease the thickness of the lens, while making the most of the characteristics of plastics with low specific gravity, a plastic material having a high refractive index has been in demand. Among materials having such performance are, for example, (1) a polymer comprising a xylylene diisocyanate compound and a polythiol compound (Japanese Unexamined Patent Publication JP-A-63-46213), (2) a resin comprising an aliphatic linear sulfur-containing difunctional isocyanate and a polythiol compound (Japanese Unexamined Patent Publication JP-A-2-153302), (3) a polymer comprising an aliphatic branched sulfur-containing difunctional isocyanate compound and a polythiol compound (Japanese Unexamined Patent Publication JP-A-10-45707).
Although the above-mentioned polymer (1) and resin (2) have an increased refractive index by limiting a combination with the polythiol compound to be polymerized, problems arise such as a decreased Abbe number and increased chromatic aberration in polymer (1) and decreased heat resistance in resin (2).
Meanwhile, the above-mentioned polymer (3) has a high refractive index, and improvements in the chromatic aberration and heat resistance are found therein, but the heat resistance is not yet satisfactory, and solvent resistance is also poor.
Furthermore, since these are uncrosslinked polymers obtained from difunctional isocyanate compounds, a special crosslinking agent is needed separately to improve the heat resistance and solvent resistance. Thus, there is a problem that it is unavoidable to limit the type of the polythiol compound to be polymerized.
It is therefore an object of the present invention to provide, under these circumstances, a novel polyisocyanate compound which can give an optical material having a high refractive index, low dispersion, excellent heat resistance and excellent solvent resistance. It is also an object and to provide a process for producing this compound at good efficiency.
It is a further object of the present invention to provide, under these circumstances, an optical material which has excellent optical characteristics such as high refractive index, low dispersion, excellent transparency, lack of optical distortion and the like, and which is also good in heat resistance, solvent resistance and weatherability. It is a still further object to provide an optical product formed of this optical material.